For many years, industrial process control systems have used varied instruments for measuring fluid pressure, especially differential pressures developed across an orifice plate in a flow conduit so as to produce a signal which is a function of the fluid flow rate. Although these various prior art devices have performed adequately, it is evident that they cannot meet all the requirements of modern industrial process control systems.
In this particular field of art, the number of prior patent disclosures is very great. Generally, the pressure transmitters of the prior art have either employed force balance or deflection measurement (i.e., motion balance) techniques to produce an electrical signal proportional to the pressure to be measured. It is this latter category to which the present invention applies.
A large number of these motion balance devices involve capacitive techniques for measuring the relative deflection of a diaphragm in response to an applied pressure. For example, U.S. Pat. No. 3,618,390 discloses a fluid-filled differential pressure transmitter having isolation diaphragms for transmitting the pressure signal to a measurement diaphragm disposed therebetween. Capacitive plates are formed on the opposing pressure chamber walls adjacent the measurement diaphragm. In this manner, the relative positioning of the measurement diaphragm in relation to the walled capacitive plates provides an output signal proportional to the applied pressure. The accuracy with which these capacitances are measured, however, depends upon the excitation frequency. Unless proper electrical isolation and additional circuitry for making the output signal independent of the applied frequency is provided, pressure transmitters that measure relative capacitance may also be subject to output errors caused by external capacitive coupling effects. Additionally, such capacitive type sensors do not provide the ready capability of remoting all active electronics from the transmitter location. This can be important in certain applications in which adjustments must be made to the electronic transducing circuitry and where the transmitter is not readily accessable.
In U.S. Pat. No. 3,277,719 a differential pressure transmitter is disclosed which operates on the principal of variable inductance, i.e, changes in differential pressure are related to the change in position of an armature which in turn is sensed by the relative inductances of two external coils. Such devices suffer from the same drawbacks as capactive type transmitters, namely their dependence on excitation frequency with an attendant need for added signal conditioning circuitry to assure accuracy, and the inability to provide simple removal of active electronic components from the transmitter.
Other pressure responsive instruments, of which U.S. Pat. No. 3,894,435 is representative, employ piezo-electric or similar strain gage elements to produce resistance changes which are a function of the strain in a mechanical element that is deflected by the applied pressure. While pressure devices operating on this principal of measurement overcome the aforementioned problems associated with the frequency domain, they involve stressing by bending or similarly deforming the force sensing element to produce the desired output signal. This frequently causes zero drift problems because such stressing can produce fatigue in the force sensing element, as well as relative motion between the strain gage and the force sensing element, thereby producing zero offset errors. Furthermore, strain gage pressure transmitters produce small electrical output signals which require additional circuitry for amplification and other signal conditioning.
Although the pressure transmitters discussed above have exhibited performance capabilities suitable for their intended applications, it is apparent that the need still exists for a pressure transmitter that is simple in construction, yet highly accurate and reliable to measure pressures under widely varying conditions.